1. Field of the Invention
This invention relates, in general, to methods and devices for non-destructive testing of opaque articles to detect surface and sub-surface cracks, flaws, voids, etc.
2. Description of the Prior Art
Various methods have been proposed to detect surface and subsurface cracks, flaws, voids, etc., in opaque solids. One common method utilizes photo-acoustic techniques in which periodic, localized heating of a sample within a gas-filled cell is caused by focused intensity modulated light, electro-magnetic radiation or a particle beam. The heat generates sound within the gas medium which is detected by a transducer, such as a microphone mounted within the gas cell. The transducer or microphone generates electrical signals which are analyzed to locate surface and subsurface defects.
In actual use, an argon-ion laser whose output is modulated is focused onto the surface of interest through an optical window spaced from the surface of the sample by a small volume of air or gas. The transducer mounted within the cell detects the amplitude and phase of pressure variations with the cell caused by the temperature profile at the surface of the sample. However, while this imaging technique is effective at detecting certain crack orientations, it cannot detect strictly vertical, closed cracks. While in practice many cracks are not quite vertical or not quite closed or both, any cracks which are strictly vertical and closed would be missed when employing this technique.
Mirage effect thermal wave imaging has proven effective at detecting strictly vertical closed cracks within opaque solids. This technique utilizes a laser to probe the air just above the surface of an opaque solid which is heated by a second modulated laser. An a.c. electrical signal is produced by using a phototransistor to monitor the deflection of the probe beam in a plane parallel or perpendicular to the sample surface. Indexing of the sample underneath the heating laser beam or indexing the heating laser beam over the surface of the sample results in a series of data signals which are useful in detecting subsurface and surface cracks, flaws, voids and other defects.
Other imaging techniques currently being used or investigated include gas cell, photothermal displacement, infrared detection and piezoelectric detection.
The signals by themselves cannot yield any useful information as to the existence of surface or subsurface cracks without additional analysis. Heretofore, on-line, real time analysis techniques have been minimal for data generated by the various thermal wave imaging techniques. This lack of useful date analysis techniques has hampered the use of thermal wave imaging techniques for detecting surface and subsurface cracks in opaque solid objects.
Thus, it would be desirable to provide an analysis technique which overcomes the deficiencies in analyzing and displaying information generated during a thermal wave scan of opaque solids. It would also be desirable to provide an analysis technique for use with a thermal wave scan of opaque solids which generates a visual image of the surface and immediate subsurface of the solid illustrating any cracks, flaws, which may exist within the solid. Finally, it would be desirable to provide an analysis technique for use with thermal wave imaging of opaque solids which generates a visual image of the existence of any cracks or flaws within the sample during real time when the sample is being probed by the thermal wave scan.